Okay, something like this happened to me with another runner making a cold-call on me, but it had nothing to do with a crush.
Follow them enough to find out where they live or where they end their run.
Then drop in on them and tell them about some upcoming charity run in the area and mention that you believe a lot in that cause, and hope they will consider participating - for the cause. Tell them you see them all the time on your daily runs, and use that as a lead-in and work it from there, even doing the event if you think he might go.
The charity angle frames you in a good light and gives you a reason to approach him.
Good luck.